1. Field of the Invention
The present invention relates to a thermal printer head with an improved efficiency.
2. Description of the Related Art
Thermal heads are classified into partial-glaze type, double-partial-glaze type and through-edge type. As shown in FIG. 27, the partial-glaze type thermal head comprises a substrate 1, a partial glaze layer 2 formed on the substrate adjacent to its edge portion, having a width equal to about 300.mu.-1200.mu. and an outwardly convex configuration, a resistive film layer 3 formed over the partial glaze layer 2, common and individual electrodes 5 and 6 formed on the resistive film layer 3 at the top positions of the glaze layer 2 opposite to each other to form a heating section 4 on the top of the glaze layer 2 and a protecting film 7 covering these layers as a whole. The double-partial-glaze type thermal head is similar to the partial-glaze type thermal head except that a portion of the glaze layer 2 placed at the heating section 4 is formed into an upwardly convex configuration by glaze etching or the like, as shown by 2a in FIG. 28.
The through-edge type thermal head is one that has the glaze layer 2 and the heating section 4 formed so as to cover the edge of the substrate 1, as shown in FIG. 29. FIG. 30 shows a modification of the thermal head shown in FIG. 29, in which the edge portion of the substrate 1 is slantingly cut to provide a slope 1.sub.B adjoining the top face 1.sub.A of the substrate 1 and an edge face 1.sub.C adjoining the slope 1.sub.B and extending perpendicular to the top Face 1.sub.A. Glaze layers 2.sub.A, 2.sub.B and 2.sub.C are formed over the respective faces 1.sub.A, 1.sub.B and 1.sub.C. The heating section 4 is formed at the slope 1.sub.B.
In order to enable the printing of any rough sheet and to improve the efficiency of the thermal head, it is necessary to focus pressure onto the ink ribbon, transfer sheet and platen at the heating section. In the partial-glaze and double-partial-glaze type thermal heads, however, the engagement of the glaze layer 2 with the rubber platen 10 through the ink ribbon 8 and transfer sheet 9 at the heating section 4 will be widened and so not provide a sufficient concentration of pressure at the heating section 4, as seen from FIG. 31. Such a problem can be somewhat overcome by the through-edge type thermal head. At present, however, the through-edge type thermal head of FIG. 29 must include a substrate having a thickness equal to about 2 mm, so that the inherent advantages of the through-edge type thermal head will not be fully attained.
Being common to the production of the conventional thermal heads, a substrate for each individual thermal head must be machined at its side edge before film formation and patterning are performed. Thus, a number of thermal heads cannot be produced from a single large-sized substrate. When it is desired to provide a thermal head in which the efficiency is improved by focusing pressure onto the heating section, the production becomes troublesome and expensive, leading to an increase in the cost for one thermal head.